Hot cathode ionization gauges are the most common non-magnetic means of measuring very low pressures and the most widely used version worldwide was disclosed in U.S. Pat. No. 2,605,431 in 1952. A typical ionization gauge includes an electron source or a cathode. The electrons emitted by the electron source collide with gas atoms and molecules in an ionization volume and produce ions. The rate at which the ions are formed is directly proportional to the density of the gas (pressure at a constant temperature) in the gauge.
Two types of ionization gauges exist: hot cathode and cold cathode. The most common hot cathode ionization gauge is the Bayard-Alpert (B-A) gauge. The B-A gauge includes a heated filament (cathode) that emits electrons toward a cylindrical wire grid (anode) defining an ionization volume (anode volume). The temperature spread for most commonly used cathodes is about 1,500 degrees Celsius to about 2,200 degrees Celsius. An ion collector electrode is disposed within the ionization volume. Electrons travel from the electron source toward and through the anode, and are eventually collected by the anode. In their travel, the electrons impact molecules and atoms of gas and create ions. The ions are attracted to the ion collector electrode by the electric field within the anode volume. The pressure of the gas within the ionization volume can be calculated from ion current (Iion) generated in the ion collector electrode and electron current (Ielectron) generated in the anode by the formula P=(1/S)(Iion/Ielectron), where S is a coefficient with the units of 1/torr and is characteristic of particular gauge geometry, electrical parameters and pressure range.
The operational lifetime of a typical B-A ionization gauge is approximately ten years when the gauge is operated in benign environments. However, these same gauges fail in hours or even minutes when operated at too high a pressure or in gas types that degrade the emission characteristics of the gauge's electron source (hot cathode). Examples of such hot cathode interactions leading to decreased operational lifetime range from degradation of the electron emission properties of the oxide coating on the hot cathode to exposure to water vapor. Degradation of the oxide coating dramatically reduces the number of electrons generated by the cathode, and exposure to water vapor results in the complete burnout of a tungsten cathode.
Cold cathode gauges come in many varieties. They include the Penning, the magnetron, the inverted magnetron, and the double inverted magnetron. The cold cathode inverted magnetron ionization gauge, sometimes referred to as a glow discharge gauge, also includes a cathode and an anode; however, the cathode is barely heated, and may heat to about a twenty degree Celsius rise over ambient temperature. The initial source of electrons is by a spontaneous emission event, or by a cosmic ray. As the electrons circle about the anode, the electrons ionize gas molecules and atoms through electron impact ionization, and other electrons are released by this event. As the cathode captures the ions, a current is generated in the cathode. This current is used as an indication of gas density and pressure. The capture of ions at the cathode also releases more electrons which are contained by the crossed electric and magnetic fields and sustains the discharge. In this way, a “cloud” of electrons and ions known as a plasma is formed in the ionization volume. However, cold cathode gauges suffer from relatively large inaccuracies due to uncontrolled discharge of electrons and surface phenomena.